The present invention relates to a nonlinear feedback control apparatus for an internal combustion engine and a feedback control method for executing feedback control of the operating state of the engine so that the rotation speed of the engine becomes stable and converges to a target rotation speed.
A proposed prior-art engine control apparatus and method are based on linear control theory. The prior art thus assures the stability and responsiveness of the control. In this prior art a dynamic model of the engine including actuators and sensors is constructed by a linear approximation of the dynamic behavior of the engine. Based on the rotation speed of the model engine, the actual rotation speed of the real engine is controlled. For example, in Japanese Published Unexamined Patent Application No. S59-120751, the model of the engine is constructed using the linear approximation of the engine behavior, and system identification.
This prior-art engine control system has the following problems when the engine is constructed based on the model.
The operating state of the engine includes a warming-up state, states where the load applied to the engine is large or small, states where the rotation speed of the engine is fast or slow, and various other states. These operating states vary widely. Such a complicated behavior of the engine cannot be determined based on the behavior of the engine model.
The actual behavior of the real engine deviates from that of the engine model. When the real engine is controlled based on the engine model, the precision of the control decreases and sufficient control characteristics of the control system cannot be obtained.
To enhance the precision of the control, in another prior art, multiple models of the engine are constructed according to the various operating states of the engine. The model of behavior approximating that of the controlled engine is selected from the multiple models. However, the multiple models make the control system intricate, thus delaying the response of the system. Furthermore, changes in the control system when the selected model changes to another model cannot be predicted. This prior art cannot really work.
Since the engine model is just theoretical, and since variables representing the internal state of the engine are determined based on the engine model, the variables do not coincide with physical control quantities. Consequently, the use of the variables is limited.